Sommerfeld Theory Colloquium (ASC)

Latest Episodes:

Supermassive Black Holes: From Jets to the Event Horizon Jun 10, 2026

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Highest resolution Event Horizon Telescope (EHT) observations will probably soon tell us more about the supermassive black hole at the Galactic Centre (Sgr A*) and the cores of active galactic nuclei (AGN). It might also help to clarify the long-standing question whether the central massive objects in AGN are instead close pairs of black holes. Mergers of supermassive black hole pairs would provide the strongest gravitational wave signals. I will present examples of how we identify potential close binary black hole candidates based on the combined analysis of high resolution radio interferometric (VLBI) observations and multi- wavelength data. I will also provide an outlook on the scientific prospects with regard to future EHT-observations.

Emergent cosmology from quantum gravity: the universe as a quantum condensate Jun 10, 2026

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The construction of a quantum theory of gravity remains an open problem despite decades of efforts. In time, the very perspective on this problem evolved. From quantising General Relativity, the goal is now mostly understood to be unraveling a more fundamental microstructure of spacetime, based on non-geometric building blocks, and to show how spacetime and matter emerge as effective, approximate notions. Given some candidate building blocks, the task becomes analogous to that of extracting the macroscopic, collective behaviour of the atoms of a condensed matter system, but even more challenging since we cannot use the usual spacetime intuition and no direct observational input is available to guide theory construction. Lacking a fundamental theory of quantum gravity, existing cosmological models which have proven extremely successful in accounting for the observed features of the very early universe (via CMB data) remain without a solid foundation, having to make a number of assumptions about a physical regime (close to the big bang), where the quantum nature of gravity and spacetime is expected to be relevant. This is all the LUDWIG-MAXIMILIANS-UNIVERSITÄT MÜNCHEN SEITE 2 VON 2 more unfortunate, since the very early universe is also where any proposed quantum theory of gravity has the highest chance of finding its observational test-bed. The gap needs to be bridged. In this talk I will first of all review the basic aspects of the problem of quantum gravity, and of some current approaches. I will then focus on one specific formalism for quantum gravity, so-called group field theories (strictly related to a number of other modern approaches). I will introduce its main features, trying to clarify the nature of the suggested building blocks of spacetime and their mathematical description. Next, I will outline a general strategy to extract an effective cosmological dynamics from quantum gravity, within this formalism. In this setting, the universe emerges as a quantum condensate of the fundamental “atoms of spacetime”, and cosmology is its corresponding hydrodynamics. Finally, I will summarize the recent results obtained along this research direction.

From Emergent Gravity to Dark Energy and Dark Matter Jun 10, 2026

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The observed deviations from the laws of gravity of Newton and Einstein in galaxies and clusters can logically speaking be either due to the presence of unseen dark matter particles or due to a change in the way gravity works in these situations. Until recently there was little reason to doubt that general relativity correctly describes gravity in all circumstances. In the past few year insights from black hole physics and string theory have lead to a new theoretical framework in which the gravitational laws are derived from the quantum entanglement of the microscopic information that is underlying space-time. An essential ingredient in the derivation is of the Einstein equations is that the vacuum entanglement obeys an area law, a condition that is known to hold in Anti-de Sitter space due to the work of Ryu and Takayanagi. We will argue that in de Sitter space due to the positive dark energy, that the microscopic entanglement entropy also contains also a volume law contribution in addition to the area law. This volume law contribution is related to the thermal properties of de Sitter space and leads to a total entropy that precisely matches the Bekenstein-Hawking formula for the cosmological horizon. We study the effect of this extra contribution on the emergent laws of gravity, and argue that it leads to a modification compared to Einstein gravity. We provide evidence for the fact this modification explains the observed phenomena in galaxies and clusters currently attributed to dark matter.

Understanding the LIGO gravitational wave event (GW150914) Jun 10, 2026

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In February 2016 the LIGO team announced the detection of gravitational waves (GW) created by the merger of two black holes. In addition to confirming a major prediction of general relativity, successful GW detection would provide a powerful new tool for astrophysics. Given their evident importance, the LIGO results and the methods which led to them deserve independent critical analysis. This talk will present the results of one such study in a manner suitable for non-specialists.

Chiral symmetry breaking, emergent Higgs mechanism, and critical matter Jun 10, 2026

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The upshot of extensive studies of �uctuations in condensed matter systems is that their qualitative importance is typically con#ned to isolated critical points of continuous transitions between phases of matter. This conventional wisdom also predicts the number of low energy Goldstone modes based on the so-called “G/H” pattern of symmetry breaking. I will discuss a class of systems, some quite wellknown, that violate this standard paradigm. Namely, they exhibit a fewer than “G/H” number of low-energy modes due to an emergent Higgs mechanism. Even more spectacularly, such systems exhibit “critical” ordered phases, with universal power-law properties reminiscent of a critical point, but requiring no #ne-tuning and extending throughout the ordered phase. One exciting recently discovered state is the heliconical nematic that in addition to above phenomena also exhibits spontaneous chiral symmetry breaking.

The Physics of Active Matter Jun 10, 2026

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Over the past ten years, there has been a growing interest among physicists for ‘active matter’, a codename that encompasses systems in which energy is taken from the environment to generate self-propulsion at the single particle level. Active particles, such as run-and-tumble bacteria, self-diffusiophoretic colloids or actin filaments in motility assays, are strongly out-of-equilibrium and exhibit much richer behaviours than their passive counterpart. In this talk I will review recent progresses regarding the physics of active particles. I will show how simple concepts like pressure, the force density exerted by assemblies of particles on their container, play a new role for active systems because of the lack of equation of state. I will also show how new collective phenomena emerge, from the transition to collective motion to the existence of cohesive matter without cohesive forces, that have no counterpart in thermal equilibrium.

Scattering Amplitudes from Geometry Jun 10, 2026

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I will review for a general audience some recent developments in our understanding of the mathematical structure of scattering amplitudes in quantum field theory. Many of these developments involve properties that have been discovered ”experimentally”: not in actual experiments, but by carrying out a tedious calculation and then observing that the result has some remarkable hidden simplicity. I will give examples of this phenomenon, and in particular I will discuss some aspects of the geometry of the ”amplituhedron”, a geometric object that is believed to completely encode certain scattering amplitudes.

Can a quantum computer solve optimization problems more Efficiently than a classical computer? Jun 10, 2026

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In this talk I will discuss connections between the physics of complex systems such as spin glasses and attempts to solve optimization problems by ”Adiabatic Quantum Computing” (AQC), a version of ”Quantum Annealing” (QA). An optimization problem is one in which one has to minimize (or maximize) an energy function in which there is competition between different terms so no single configuration of the variables minimizes each term in the energy. In statistical physics this competition is called ”frustration”. It leads to a complex energy ”landscape” with many valleys separated by barriers, so simple algorithms easily get trapped in local minima which have a higher energy than the global minimum. Many problems in science, and engineering are formulated as optimization problems. In quantum annealing one tries to avoid being trapped in a local minimum by adding quantum fluctuations so the system can tunnel to regions of lower energy. The strength of the quantum fluctuations is gradually reduced to zero during the annealing schedule. This method applies to problems with binary variables, known as qubits in the quantum case. There is considerable interest in AQC at present, in large part because a company, D-Wave, has produced an actual device, the latest version of which has about one thousand qubits. In addition, there has been considerable theoretical work mainly using computer simulations to see if there is a ”quantum speedup” compared with analogous classical algorithms in which thermal, rather than quantum, fluctuations are used to escape from local minima. In the talk I will discuss difficulties in obtaining a quantum speedup due to (i) (quantum) phase transitions that the system can undergo during the annealing schedule, and (ii) the sensitivity of the state of the system to the precise values of the interactions, i.e. chaos. A related chaotic effect is that the state of the system can change dramatically with small changes in the temperature (temperature-chaos), for thermal annealing, and the strength of the quantum fluctuations, for quantum annealing.

Near-Pristine Gas at High Redshifts: First Stars, Big-Bang Nucleosynthesis, and Limits on Dark Radiation Jun 10, 2026

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In this seminar, I shall describe recent work by our group on iden- tifying pockets of gas at high redshift that have undergone mini- mum processing through stars. The chemical composition of such gas still bears the imprints of the first few generations of stars that formed only a few hundred million years after the Big Bang, and thereby gives us clues to the physical properties of these still mys- terious objects which heralded the so-called ‘epoch of reionisation’. Near-pristine gas at high redshift is also the astrophysical environ- ment where the primordial abundance of deuterium can be measured most precisely. I will show how determinations of the cosmic den- sity of baryons from Big-Bang Nucleosynthesis and from the Cosmic Microwave Background have now reached comparable precision, in both cases of order of a few percent. The excellent agreement be- tween these two measures at widely different cosmic epochs places interesting limits on the existence of relativistic particles beyond the standard model of physics.

Gauge Theories and Non-Commutative Geometry Jun 10, 2026

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We shall review the attempts to extend the quantum mechanical property of non-commutativity from phase space to ordinary space. These attempts took a more precise form in the case of gauge theories for which some concrete results have been obtained. In flat space they amount to a reformulation of the theory which looks interesting but they have not given so far any novel physical results. However, the introduction of gravity gives a richer structure and may offer some new insights.